Thermal Properties of Copolymer Gels Containing N-Isopropylacrylamide Mitsuhiro Shibayama,* Shin-ya Mizutani, and Shunji Nomura Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan Received September 15, 1995; Revised Manuscript Received December 8, 1995 X ABSTRACT: The phase transition of gels containing N-isopropylacrylamide (NIPA) was investigated by differential scanning calorimetry (DSC) and swelling measurements. The enthalpy of dissociation of the hydrophobic interaction per molar unit of NIPA chains (ΔHN), the transition temperature (Tc), and the number of water molecules associated with an NIPA monomer, (n) were evaluated as a function of polymer concentration. Significant differences in ΔHN and Tc were found between two systems: poly(NIPA-ran- acrylic acid) (NIPA/AAc; a weakly charged gel) and poly(NIPA-ran-dimethylacrylamide) (NIPA/DMAA; a neutral gel). ΔHN decreases with increasing comonomer concentration. However, a larger decrease in ΔHN was observed for NIPA/AAc than for NIPA/DMAA, which is accounted for by the strong hydrophilic effect of the charged AAc comonomers. No noticeable copolymer concentration dependence in n was observed in both systems. It is suggested that there are two types of water molecules, i.e., one associated with the phase transition, (n - n0), and the other the lower limit for the hydrophobic hydration, n0. The roles of these water molecules are discussed in relation to the volume phase transition. Introduction It is well known that poly(N-isopropylacrylamide) (PNIPA) gels undergo a volume phase transition in water at the transition temperature, T c , from a swollen state to a shrunken state by increasing temperature. 1,2 This is due to dissociation of the hydrophobic interaction between NIPA segments and water. By copolymerizing NIPA with acrylic acid (AAc), one can obtain a gel which has a higher T c and a larger swelling power at T < T c . 3 This phenomenon is well elucidated by microscopic structure investigations by means of small-angle neu- tron scattering (SANS). 4 The SANS analysis disclosed an interesting feature of concentration fluctuations. Above the Θ temperature of PNIPA, which is about 34 °C, strong concentration fluctuations of the order of a few tens of nanometers appeared. These concentration fluctuations grew with increasing temperature until a macroscopic shrinking transition took place. This phe- nomenon was explained with two antagonistic interac- tions, i.e., electrostatic interaction and hydrophobic interaction. Not only in polymer science but also in other fields of science, studies of hydrophobic interaction are of great significance. For example, many kinds of foods, such as gluten and fish paste, are materials coagulated with hydrophobic and/or hydrogen-bonding interactions. Therefore, control of the hydrophobic interaction is one of main themes in food science and/or processing. 5 PNIPA gels are regarded as typical model systems to study the mechanical and thermal properties as well as processability of these kinds of food. 5,6 Thermal analysis of the demixing transition of linear PNIPA’s was first reported in the literature by Heskins and Guillet in 1969. 7 Shild and Tirrell reported that the transition was found to be independent of heating rate of e30 °C/h. 8 However, they observed a large difference in the shape and height of the endotherms in PNIPA’s having different molecular weight distribu- tions. In the case of PNIPA gels, Otake et al. reported the enthalpy change related to the volume phase transi- tion 9,10 and proposed a theory for the transition. 9 In a previous paper, 11 we employed differential scan- ning calorimetry (DSC) to investigate the hydrophobic interaction of poly(NIPA) and found that about 750 cal/ mol of NIPA monomer units is required for the dissocia- tion of the hydrophobic interaction and about 13 water molecules are released from an NIPA monomer unit. In the present paper, we extend this analysis to two types of copolymers: poly(NIPA-ran-acrylic acid) (NIPA/ AAc; a weakly charged gel) and poly(NIPA-ran-di- methylacrylamide) (NIPA/DMAA; a neutral gel). Experimental Section Samples. N-Isopropylacrylamide (NIPA) and N,N-di- methylacrylamide (DMAA) monomers were kindly supplied by Kohjin Chemical Co. Ltd., which were purified by recrystal- lization. Acrylic acid of special grade (AAc) was purchased from Wako Chemical Co. and used without further purifica- tion. The prescribed amounts of the monomers were dissolved in deionized water and then mixed with N,N′-methylenebis- (acrylamide) (BIS) (cross-linker) and ammonium persulfate (APS) in deionized water. The polymerization was initiated with N,N,N′,N′-tetramethylethylenediamine (TEMED) at 20 °C after degassing the pregel solution. The concentrations of these reagents were 22.4 (BIS), 1.75 (APS), and 8 mM (TEMED) for NIPA/DMAA, which were the same as those employed in the previous paper. 11 In the case of NIPA/AAc gels, the concentrations were 8.6 (BIS), 1.88 (APS), and 32 mM (TEMED), which are typical concentrations for the preparation of acrylamide gels for electrophoresis except for the TEMED concentration. The TEMED concentration was raised so as to keep a basic atmosphere in the presence of AAc. Table 1 shows the sample codes as well as the compositions for the NIPA/DMAA and NIPA/AAc copolymer gels. The total mono- mer concentrations were fixed to be 700 mM. Gels prepared in micropipettes were washed with deionized water (pH ≈ 5.6). Special care was paid for washing the gel since it affected the transition temperature, particularly in the case of the NIPA/ AAc copolymer gels. 4 Shrinking Curve Measurements and DSC. The shrink- ing curve was obtained with a Nikon inverted microscope coupled with an image analyzing system, which allowed us to precisely determine the gel diameter with an error of less than (5 μm. DSC measurements were carried out with a DSC3100 (Mac Science Co. Ltd.). The polymer concentration in a gel was adjusted by deswelling the gel at T > Tc. Gels having * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, February 1, 1996. 2019 Macromolecules 1996, 29, 2019-2024 0024-9297/96/2229-2019$12.00/0 © 1996 American Chemical Society